Originally Posted by
Beech19
Thanks Ex P, that makes sense to me.
But with all engines running, is your thrust to weight ratio approximatly the same on a twin as it is on a four engined?
Higher thrust to weight ratio for a two engine aircraft compared to four engine. If 100,000 pounds of thrust is needed for a 700,000 pound aircraft to meet the climb requirements (FAR 25.121) in case of loss of one engine then each of the engines would have to be rated at 100,000 pounds giving the aircraft 200,000 pounds of total thrust. The 777-300ER has 230,600 pounds of total thrust (115,300 each) and has a max gross weight of 759,000 Ibs.
But if the same weight class example aircraft has four engines then it would only need 133,000 pounds of total thrust, so if one engine is lost on takeoff (33,000 pounds of thrust) then it would still have the 100,000 pounds of thrust available to meet the performance criteria. The 747-100 has 184,000 Ibs of total thrust (46,000 Ibs each) and has a max gross weight of 735,000 Ibs.
The comparison of these two aircrafts' power to weight ratios shows the differrence.
There's no requirement or guarantee for a transport category aircraft to lose more than one engine on takeoff and be able to continue if at max takeoff weight. There are enroute two engine out requirements that require slightly more thrust than what I used in my example four engine aircraft, and there are ETOPS requirements and considerations which raise the two engine aircraft thrust even higher than what's needed for the one engine out takeoff climb such as ability to maintain higher altitudes on one engine for fuel burn range reasons.